Terahertz radiation is electromagnetic waves that have a frequency between 100 GHz and 30 THz, lying between the infrared and microwave parts of the spectrum. The radiation is non-ionizing and can penetrate most non-metallic objects but is absorbed by polar materials and liquids. Consequently, terahertz technology provides a number of spectroscopy and imaging applications, and is a fast-growing field.
Terahertz pulses are distorted by passing though various materials including gases, liquids, and solids. It is well known that different materials alter the terahertz waves differently, depending on the material and the frequency content of the signal. It is the purpose of terahertz signal processing to detect and classify these changes. Depending on the application, some of the changes are undesired and must be compensated for.
In terahertz signal processing a detected signal often contains several echoes of the same signal due to reflections of the signal. Depending on the setup, reflections can come from sample edges, wave-guide ends, the terahertz source structure, the terahertz detector structure or any of a number of other sources. In some cases, multiple reflection mechanisms may be combined.
The simplest way to handle the reflection mechanisms in terahertz signal processing is to time-gate the signal before the occurrence of any echoes due to the reflections. However, by time-gating the signal, the frequency resolution of the signal is also decreased since it is inversely proportional to the time-length of the signal. Reduced frequency resolution results in a decreased image resolution in imaging applications, and in spectroscopy applications, may result in failing to detect spectroscopic indicators with a narrow frequency response.
Other approaches to minimize the echo effect have attempted to solve the problem with the hardware setup. However, due to the inherent nature of terahertz systems this approach is practically impossible, or at the least, costly in terms of the system complexity and terahertz signal quality.
Presently, the most practical solution involves taking an extra reference measurement without the sample to be measured present. The reference measurement is then differentiated or deconvolved from the main measurement to remove the reflection effects. However, in practice this may not be possible or easily accomplished. For example, if the reflection is coming from the sample to be measured, one could remove the sample and replace it with another object which generates exactly the same reflection effect. In spectroscopy, where the sample is unknown, or in cases where the sample is structurally complex, it is either impossible or very difficult to replace the sample without introducing other effects. Taking an extra reference measurement also does not account for the reflections within the structures of the terahertz emitter and detectors themselves. Also, temperature fluctuations, change in beam position, or other factors affecting laser stability between the reference measurement and sample measurement can introduce errors.
Accordingly, there is a need for improved signal processing in terahertz spectroscopy and imaging applications that can remove the echo effects without using a measured reference signal.